1. Field of the Invention
The present invention relates to determining the relative amounts of phases in multiphase flow, typically the liquid (or gas) fraction of a stream in two-phase flow. More particularly, the present invention relates to quickly and reliably determining the liquid (or gas) fraction of a flowing two-phase stream, such as a process stream commonly found in chemical processing or in petroleum production operations. The present invention yields accurate results regardless of the exact flow regime under which the two-phase stream is flowing.
2. Description of the Prior Art
It is often important to have a reliable on-line quantitative measurement of the relative amounts of the various phases present in a stream containing multiple phases. Such streams are often encountered in the context of a petroleum refinery, a chemical plant, or in the production of petroleum from underground reservoirs. Many such streams typically contain multiple phases. A measurement of the relative amounts of the various phases is not only important in determining the amounts of liquid and gas in a custody transfer context, but is also often crucial in the context of chemical process operations in allowing the operators and/or the automatic control equipment to adjust process parameters so as to optimize the operation and avoid unprofitable and/or dangerous operating conditions.
Various attempts have been made to determine the relative amounts of the various phases in multiphase flow. The best methods have been found to be those involving radiation attenuation techniques. For this purpose, gamma rays or x-rays have been often used.
Generally, radiation attenuation techniques involve the irradiation of a small section of the pipe through which the stream to be measured is flowing. The radiation enters the pipe by passing through the pipe wall on the side closer to the radiation source. The radiation then passes through the fluid in the pipe. Finally, the radiation exits the pipe by passing through the pipe wall on the side away from the radiation source.
As the radiation passes through the various materials, some of the radiation is absorbed and de-energized. Thus, as the stream of radiation passes through the various substances in its path, the amount of radiation continuing on a straight path from the source is decreased, or attenuated. The degree of attenuation depends on the properties of the materials through which the radiation passes and on the amounts of those materials present in the path of the radiation beam. The amount of radiation surviving its passage through the various materials in its path is then detected by appropriate radiation detectors.
An advantage of the radiation attenuation method is that the apparatus can be mounted completely outside the pipe and thus does not in any way interfere with the stream being analyzed.
Of great interest in the measurement of multiphase streams is the instantaneous liquid (or gas) fraction across the pipe. It is known to use an apparatus for determining the liquid (or gas) fraction which uses a radiation source to direct a single beam of radiation at a pipe, and to detect the attenuated radiation beam exiting the pipe. However, it is well known that the liquid (or gas) is not homogeneously distributed across the pipe cross-section. Thus, use of a single beam apparatus can lead to large errors in the determination of the overall liquid (or gas) fraction.
A number of solutions have been attempted in an effort to overcome this problem. One such solution is to cause the single radiation beam to traverse the entire pipe cross section and to take an appropriate weighted average of the attenuated beam in determining the average liquid (or gas) fraction. This technique suffers from at least two shortcomings. First, the method requires movement of the radiation beam which introduces considerable mechanical complexities to the system and reduces system reliability and ruggedness. Second, since a certain finite time is required for the radiation beam to traverse the entire pipe cross section, the average liquid (or gas) fraction calculated reflects the average fraction over a certain small but finite period of time, rather than an instantaneous real-time reading of the liquid (or gas) fraction.
To overcome the difficulties associated with single radiation beam techniques, work has been done to develop a one-shot-collimator measurement, also known as the wide beam measurement. The wide beam technique calls for the simultaneous bombardment of the entire cross section of the pipe with a beam of radiation emanating from a single source, and for the simultaneous detection of the attenuated radiation exiting the pipe. This arrangement provides an instantaneous response which is representative of the average liquid (or gas) fraction of the stream in the pipe. The average reading is obtained by summing the attenuations of the various portions of the radiation beam. Such a technique is described, for example, in "On the Gamma-Ray One-Shot-Collimator Measurement of Two-Phase-Flow Void Fractions", R. P. Gardner, R. H. Bean, and J. K. Ferrell, Nuclear Applications & Technology, Vol. 8, January 1970, pp. 88-94.
While the wide beam technique described above is capable of yielding accurate measurements for two-phase flows wherein the two phases are intimately mixed, large errors are obtained when the technique is used to determine liquid (or gas) fractions in two-phase systems wherein the two phases are relatively distinct and separate from one another, such as is the case where, for example, the phases are in annular flow with gas flowing through the central region of the pipe with the liquid flowing in an annular area near the pipe walls, or in stratified flow, where the liquid tends to concentrate near the bottom portion of the horizontal or inclined pipe while the gas tends to locate toward the upper portions of the pipe cross section.